1. Field of the Invention
This invention relates to methods and systems for determining the wavelength of light transmitted through an optical fiber. In a particularly preferred embodiment, the invention also relates to methods and systems for determining the wavelength of light transmitted through an optical fiber which incorporate a fiber grating with a variably modulated index of refraction.
2. Description of Related Art
It is often useful to be able determine the wavelength of light transmitted through an optical fiber. Many types of fiber optic sensors are known which alter their transmissivity in accordance with a sensed parameter. For example, fiber optic Bragg grating temperature sensors and pressure sensors are known which are generally transmissive but which reflect light at a wavelength that varies according to the temperature or pressure sensed by the sensor. By determining the wavelength of light which is reflected by such a sensor, an indication may be obtained of the temperature or pressure sensed by the sensor.
In practice, the reflection from the sensor consists of light within a narrow-banded spectrum that is centered about an average wavelength. Therefore, the wavelength that is determined is typically the average wavelength of the reflection. In general, however, any wavelength that corresponds to the Bragg reflection and that varies in accordance with the sensed parameter may be determined in order to obtain the temperature or pressure information. For convenience, the wavelength that is determined will now be referred to as the average wavelength, it being understood however that the determined wavelength could also be a wavelength other than the average wavelength.
Various demodulation techniques are known for determining the wavelength of light transmitted through an optical fiber. According to one particular technique, a dispersive coupler is used to split light from an input fiber between two output fibers. Since the coupler is dispersive, light is transmitted to the two output fibers in a manner which is wavelength-dependent. In particular, the coupler has a sinusoidal transmissivity function. Thus, light at wavelengths occurring at maxima of the sinusoid is coupled to one output fiber, light at wavelengths occurring at minima of the sinusoid is coupled to the other output fiber, and light at wavelengths occurring in between maxima and minima is coupled in varying proportions to both optical fibers. Neither output fiber receives all of the wavelengths contained in the light at the input fiber.
The two output fibers are coupled to two photodetectors which respectively measure the intensity of light in each of the two output fibers. The ratio of the total intensity of light in each of the two output fibers is then determined. Given this ratio, the average wavelength of the light can be determined using the known transmission characteristics of the dispersive coupler (namely, the variation of the transmissivity of the dispersive coupler as a function of wavelength). For example, if the ratio of the two intensities is approximately equal to one, then it is known that the average wavelength of the light is at about the middle of the operating spectrum of the dispersive coupler.
This approach has two disadvantages, however. The first disadvantage of this approach is that the relationship between the ratio of the intensities in the two output fibers and the average wavelength of the light is non-linear. As a result, the sensitivity of the demodulation system is greater at some wavelengths and less at other wavelengths. Additionally, the fact that the relationship is non-linear complicates the determination of the average wavelength.
The second disadvantage of this approach is that its dynamic range is limited. In order to be able to uniquely relate a given intensity ratio to a particular average wavelength, this technique can be used only within a range of wavelengths that fits within one half of a period (at most, less in practice) of the sinusoidal transmission function of the dispersive coupler. However, the range of wavelengths which can be fit into one half of the period of the sinusoidal transmission function is limited by the physics behind the construction of the dispersive coupler.
Thus, what is needed is an improved method and system for determining the average wavelength of light transmitted through an optical fiber. In particular, what is needed is a method and system in which a linear relationship exists between measured intensity and the average wavelength of the light. What is also need is a method and system in which the range of wavelengths which can be demodulated is much larger than in existing systems.